Dihalobutenediol containing urethane foam compositions

ABSTRACT

Polyol blends which comprise a mixture of dihalobutenediol and polyhydroxy compounds wherein the hydroxyl number of the mixture is from about 200 to 800 are disclosed. A process for preparing polyol blends and flameretardant polyurethane compositions using the polyol blends are also disclosed.

This application is a division of our copending U.S. patent applicationSer. No. 123,109, filed Mar. 10, 1971, now U.S. Patent No. 3,812,047.

This invention relates to novel polyol blends, to a process forpreparing these compositions, and to flame-retardant polyurethane foamcompositions prepared from these polyol compositions. More particularly,this invention relates to polyol blends and polyurethane foamcompositions which contain dihalobutenediol and to a process forpreparing a blend of dihalobutenediol and a polyhydroxy compound. Still,more particularly, this invention relates to flame-retardant foamsderived from polyol blends containing dihalobutenediol and to theparticular polyol blends used in the preparation of the foams.

Polyurethane foams, for reasons of economy and ease of handling, havebecome widely used as insulating materials in the construction andmanufacturing arts. Notwithstanding that polyurethane and similarlyconstituted foams are excellent insulating materials by reason of theircharacteristic property of a low specific heat, this property itselffrequently causes such foams to build up heat excessively within thesurface layers thereof, which may result in the ignition of the foams.In the past, efforts have been made to overcome the disadvantage arisingfrom the use of polyurethane and similar foams in the construction andother industries by adding to such foams a variety of additives, such asphosphorus and halogen-containing compounds, which tend to inhibit theflammability thereof. Frequently, however, the use of additives of theaforementioned kind results in difficulties in the preparation of theflame-retardant polyurethane foams arising from compatability problemswith the reactant materials. Therefore, polyurethane foams prepared fromreactant materials which would impart the characteristic offlame-retardancy thereto without the inclusion of possible incompatibleadditives represents a desirable advance in the art.

It is, accordingly, an object of this invention to provide a novelfire-retardant polyurethane foam composition.

It is another object of this invention to provide a novel polyol blendfor use in said flame-retardant polyurethane foam compositions.

It is another object of this invention to provide novel processes forthe preparation of said polyol blends. Still other objects of thisinvention will become evident to those skilled in the art from thefollowing detailed description of the invention.

Broadly, the polyol blends of this invention are mixtures ofdihalobutenediol and organic polyhydroxy compounds wherein the hydroxylnumber of the resulting polyol blend is from about 200 to about 800 andthe weight percent of the dihalobutenediol in said blend is at leastabout 10%.

In a class of polyol blend of this invention the weight percent of2,3-dihalo-2-butene-1,4-diol based on the blend of diol and polyhydroxycompound does not exceed 60%. In a preferred polyol blend the weightpercent of the 2,3-dihalo-2-butene-1,4-diol is from about 15 to about40% based on the total weight of the diol and polyhydroxy compound andthe hydroxyl number of the diol and polyol blend is from 300 to 600.

Among the 2,3-dihalo-2-butene-1,4-diols used in accordance with thisinvention are dibromobutenediol, dichlorobutenediol, anddiiodobutenediol. The preferred diol is dibromobutenediol.

Exemplary of the polyhydroxy compounds which can be used in conjunctionwith the dibromobutenediol to prepare polyol blends within the polyolblends of this invention are polyhydroxy alkanes, alkoxylatedpolyhydroxy alkanes, sugars, alkoxylated sugars, hydroxyl terminatedpolyesters, hydroxyl terminated aromatic polyethers, and methylolated oralkoxylated amines.

More particularly within the category of polyhydroxy alkanes areincluded sugar alcohols, and glycols: for example, ethylene glycol,propylene glycol, butanediol, sorbitol, glycerine, erythritol, threitol,and 1,2,5,6-hexanetetrol.

The alkoxylated polyhydroxy alkanes include the reaction product of analkylene oxide or arylene oxide with a polyhydroxy alkane within theabove-described class. The oxide which can be used in preparing theoxide derivatives can be any which is reactive with the polyhydroxyalkane without excessive degradation occurring. Among the oxides withinthis class are ethylene oxide, propylene oxide, butene oxide, andstyrene oxide. The mols of oxide per hydroxyl group of polyhydroxyalkane are usually from 1:1 to about 20:1. Included as a polyhydroxyalkane are those polyethers represented by the formula

    HO --R -- O-- H

where R is an alkenyl radical or an aryl substituted alkenyl radical andsaid alkenyl radical contains up to 4 carbon atoms.

The hydroxyl terminated aromatic polyethers which may be used as part ofthe polyol blend of the invention include polyphenyl ethers, and thereaction product of aromatic diols such as bisphenol A and an alkyleneoxide. In all cases the aromatic polyethers have an ether linkagedirectly to the aromatic nucleus. Examples of the polyethers arepolyoxyethylene (10)-2,2-bis(4-hydroxyphenyl)propane,polyoxystyrene(40)bis (2,6-dibromo-4-hydroxyphenyl)ethane and ##SPC1##

The sugars contemplated include mono- and disaccharides such as glucose,fructose, maltose, sucrose and mannose. The alkoxylated sugars includesugars containing up to about 10 oxyalkylene or oxyarylene groups perhydroxyl group of said sugar. Examples of these alkoxylated sugarsinclude polyoxystyrene(10) sucrose, polyoxybutylene(6)glucose,polyoxyethylene(8)maltose.

The hydroxyl terminated polyesters which can be used are those hydroxylterminated polyesters which are liquid at room temperature and mayinclude the reaction products of saturated or unsaturated diacids withdihydroxy compounds. Examples of the acids include phthalic, succinic,adipic, fumaric, maleic, and bis(4-carboxyphenyl)propylene. Thedihydroxy materials include diols such as butanediol, hexanediol,tetrabromobisphenol A, bis(4-hydroxyphenyl)ethylene, dibromobisphenol A,and alkoxylated derivatives of such dihydric materials.

Amines which can be used as hydroxyl bearing compounds are methylolatedand alkoxylated aliphatic and aryl amines. More particularly,methylolated and alkoxylated amines which can be used in preparing thepolyol blends of this invention include methylolated guanamines,methylolated melamine, alkoxylated melamines and guanamines, andmethylolated and alkoxylated diamines and monoamines which contain atleast one primary amine group. Examples of these amines includepolyoxyethylene(7)-2,4-diamino-6-phenyl-s-triazine (more commonly calledpolyoxyethylene (7)benzoguanamine);2,4,6-tris[bis(hydroxymethyl)-amino]-s-triazine; N,N-bis(hydroxymethyl),N'-methyl, tetramethylene diamine;N,N,N',N'-tetrakis(hydroxymethyl)pentamethylene diamine;N,N-bis(hydroxymethyl) N'-ethyl, N'-propyl, tetramethylene diamine; andN'N'-bis(hydroxymethyl)propylamine.

The polyol blend of this invention may be prepared in various ways. Thepolyol blend may be prepared by mixing 2,3-dihalo-2-butene-1,4-diol witha polyhydroxy compound wherein the nature and amount of the polyhydroxycompound is chosen to achieve a polyol blend having a hydroxyl number ofbetween about 200 and about 800. A preferred process for preparing thepolyol blend of this invention comprises mixing 2-butyne-1,4-diol withan amine free polyhydroxy compound in a ratio so that at least about31/2 weight percent of the mixture is 2-butyene-1,4-diol and addinghalogen, e.g. bromine, to the mixture at a temperature of less thanabout 100°C. and in a molar ratio sufficient to substantially halogenatethe 2-butyne-1,4-diol to 2,3-dihalo-2-butene-1,4-diol. The final mixtureof dihalobutenediol and the polyhydroxy compound will have a hydroxylnumber of about 200 to about 800. The reason the temperature of thisreaction is generally maintained below 100°C. is not because of lack ofhalogenation potential at temperatures above 100°C. but rather that attemperatures exceeding 100°C. undesirable side reactions may occur whichmay limit the utility of the final polyol blend. More particularly, themolar ratio of halogen to butynediol during the course of thehalogenation reaction may vary from about 0.9 to 1.2 and the temperatureof the reaction may be from -5°C. to 25°C.

The polyol blend prepared according to the above preferred process willbe a relatively stable blend of the dibromobutenediol and the particularpolyhydroxy compound which constitutes the second member of the blend.Naturally, mixtures of more than one polyhydroxy compound can be used inpreparing the blends of this invention. However, as indicated above,polyol blends containing methylolated or alkoxylated amines cannot beused in the in situ halogenation and thus polyol blends containing amethylolated or alkoxylated amine are prepared by alternate processes.

The following are representative examples of polyol blends which arewithin the scope of this invention and the process for preparing suchblends:

EXAMPLE 1

43.5 grams of 2-butyne-1,4-diol are dissolved in distilled water at roomtemperature. To this solution of butynediol, 81.5 grams of bromine areadded dropwise over a period of 30 minutes. During the addition of thebromine to the butynediol solution the reaction vessel is cooled in awater bath and a temperature is maintained between 25° and 35°C. Thesolution during the addition of the bromine is continually stirred witha magnetic stirrer. After the entire amount of bromine is added, thesolution is stirred for an additional 15 minutes to insure the completereaction of the bromine with the butynediol. The product is aquantitative yield of 2,3-dibromo-2-butene-1,4-diol. To this homogeneoussolution is then added 375 grams of polyoxypropylene (8) sorbitol at aconcentration of 95 weight percent solids in an aqueous solution. Thismixture of polyoxypropylene(8)sorbitol and dibromobutenediol is thenstirred until homogeneous. After homogeneity is reached the solution isyellow and slightly hazy. This slightly hazy solution is then heated for15 minutes at 70°C. until it is completely clear. The aqueous solutionis then passed through a cation exchange column to remove undesirableacid by-products. Then the water is removed under vacuum at 75°C. andthe final polyol blend has a dark color and has a hydroxyl number of455.

EXAMPLE 2

43.5 grams of 2-butyne-1,4-diol, 375 grams of polyoxypropylene (8)sorbitol and 350 grams of distilled water are mixed together until ahomogeneous solution is formed. This resultant solution is then cooledto a temperature between 25° and 35°C. and over a period of 30 to 60minutes 81.5 grams (37.85 ml) of bromine is added with stirring. Afterthe completion of the bromine addition, the solution is stirred for anadditional 15 minutes to insure complete reaction. The resultantreaction mixture is then cation exchanged and finally the water isremoved under vacuum at a temperature from between 70° and 75°C. Thepolyol blend has a hydroxyl number of 480 and an acid number of 8. Thefinal color of the polyol blend is a dark brown.

EXAMPLE 3

According to the procedure of Example 2, a polyol blend is prepared from101/2 grams of 2-butyne-1,4-diol, 100 grams ofpolyoxyethylene(6)sorbitan, 70 grams of polyoxypropylene(10) glycerineand 291/2 grams of bromine. The hydroxyl number of this polyol blend is337.

EXAMPLE 4

According to the procedure of Example 1, 100 grams of2,3-dibromo-2-butene-1,4-diol in an aqueous solution and 100 grams ofpolyoxypropylene diethylene triamine are dissolved in 200 grams ofwater. After cation exchanging the final polyol blend has a hydroxylnumber of 462.

EXAMPLE 5

60 grams of a polyester, prepared by heating 177.9 grams of glycerineand 416.2 grams of phthalic anhydride in the presence of potassiumhydroxide catalyst to 140°C. and then adding 288 grams of propyleneoxide, is blended with 40 grams of dibromobutenediol in an aqeuoussolution according to the procedure of Example 1. The final blend ofpolyol has a hdyroxyl number of 433.

EXAMPLE 6

According to the procedure of Example 1, 100 grams of dibromobutenediolin an aqueous solution, 40 grams of sorbitol, and 60 grams ofpolyoxypropylene(10) sorbitol are blended and cation exchanged. Thefinal polyol blend has a hydroxyl number of 730.

EXAMPLE 7

According to the procedure of Example 1, 60 grams of2,3-dichloro-2-butene-1,4-diol, 20 grams of glycerol, and 120 grams of apropylene glycol polyether having an average molecular weight of 500 areblended in a mixer. The resulting polyol composition is cation exchangedand found to have a hydroxyl number of 500.

In accordance with the present invention, the above-described polyolcompositions may be used to prepare flame-retardant polyurethane foamcompositions comprising an organic multifunctional isocyanate and anabove-described polyol composition.

In general, the foams of this invention are prepared by reacting apolyol blend in accordance with this invention with an organicmultifunctional isocyanate in a ratio suitable to provide from about 0.8to 1.3 isocyanate groups per hydroxyl group of said polyol blend. Apreferred ratio of isocyanate groups to hydroxyl groups is from about0.95 to about 1.05.

The conditions under which the urethane foam is prepared are notcritical. Temperatures as low as -40°C. may be used if proper catalystconcentration is used and if the mixture does not become too viscous.The upper temperature is limited only by the volatility of thereactants. A practical temperature range is from about 0° to about 60°C.The process can be carried out by both continous and batch processingtechniques.

A wide variety of organic isocyanate compounds may be used to preparethe novel polyurethane foam compositions among which are includeddiphenyl diisocyanate; toluene diisocyanate;chloro-phenyl-2,4-diisocyanate; 1,4-tetramethylene diisocyanate;p-phenylene-diisocyanate; 3,3-dimethyl-4,4'-phenylene diisocyanate;3,3'-dimethoxy-4,4'-biphenylene diisocyanate; polymethylene polyphenylpolyisocyanate (PAPI, which is an essentially tri-functional, darkcolored liquid); methylene bis (4,4'-phenyl polyisocyanate) (Mondur MR,which is primarily diphenyl methane 4,4'-diisocyanate, but has anaverage isocyanate group content of about 2.5 per molecule); and otherpolymethylene polyphenyl isocyanates containing in average of from about2.2 through 3.3 NCO groups per molecule.

Catalysts which may be used in the preparation of polyurethane foams ofthis invention may include amine catalysts and tin catalysts or mixturesthereof. Among the suitable amine catalysts are N-alkyl morpholines suchas N-methyl morpholine and N-ethyl morpholine, tertiary amines such astrimethyl amine, triethyl amine, tetramethyl guanidine,triethylenediamine, N,N,N',N'-tetramethyl-1,3-butanediamine; dimethylethanolamine, piperazine and piperazine derivatives, such as N-methylpiperazine, and 1,4-diazabicyclo(2.2)octane. Amine catalysts may bepresent in amounts from about 0.05% to about 2.0% by weight based on theweight of the reaction mixture. Among the suitable thin catalysts areincluded dialkyl tin laurates (such as dibutyl tin dilaurate), dibutyltin-di-2-ethyl hexoate, dibutyl tin diacetate, stannous oleate, andstannous octoate. Tin catalysts may be present in amounts from about0.01% to about 1.0% by weight based on the weight of the reactionmixture.

In a preferred embodiment of preparing the polyurethane foamcompositions, a surface active agent and a foaming agent are used.Examples of useful surface active agents which can be present in amountsof from about 0.05% to about 2% by weight of the hydroxy-bearingcompounds used to prepare the polyurethane foams of this invention arewater-soluble siloxane-oxyalkylene block copolymers as described in U.S.Pat. No. 2,834,748 to Bailey et al., issued May 13, 1958. A typicalorgano-silicon surfactant is L5310 available from Union Carbide Co.Other surfactants which may be used are condensates of ethylene oxidewith a hydrophobic base formed by condensing propylene oxide withpropylene glycol. These surfactants have a molecular weight within therange of about 2000 to about 8000 and are generally ascribed the formula

    HO(C.sub.2 H.sub.4 O).sub.a (C.sub.3 H.sub.6 O).sub.n (C.sub.2 H.sub.4 O).sub.c H.

"l-5320 silicone" is the Union Carbide Company's propriatary name forits low viscosity organo-silicone having a hydroxyl number ofapproximately 170 and which is a liquid organo-silicone surfactant. Itssolid counterpart is designated to the trade as "L-5310" and is coveredby British Patent No. 1,015,611.

Dow Corning 195 Surfactant is a siliconeglycol copolymer having thefollowing typical properties:

    Viscosity at 77 f, centistokes                                                                        325                                                   Specific Gravity at 77 F                                                                              1.07                                                  Refractive Index at 77 F                                                                              1.4540                                                Color, Gardner Scale    4                                                     Hydroxyl Content (Phthalic                                                    Anhydride Method) percent                                                                             1.8                                               

Another class of surfactants comprises alkylene oxide adducts ofethylene diamine having generally the formula: ##EQU1## Still anotherclass of surfactants comprises the polyoxyalkylene esters of long chainfatty acids and sorbitan such as polyoxyethylene(20)sorbitanmonolaurate, polyoxyethylene(4)sorbitan monolaurate,polyoxyethylene(20)sorbitan tristearate, polyoxyethylene(20)sorbitanmonooleate, polyoxethylene(5)sorbitan monooleate andpolyoxyethylene(20)sorbitan trioleate.

Although the polyurethane foam compositions of this invention may usewater as a foaming agent, a halogenated saturated aliphatic hydrocarbonor a mixture of such halogenated saturated aliphatic hydrocarbons ispreferred, for example, trichlorofluoromethane (Freon 11);monochloroethane; monochloromonofluoroethane; 1,2-dibromo,1,1,2,2-tetrafluoroethane; 1,1,2-trichloro 1,2,2-trifluoroethane;1,1,2,2-tetrafluoro 1,2-dichloro-ethane;1,2-difluoro-1,1,2,2-tetrachloroethane; dichloromethane; dibromomethaneand their mixtures. These materials may be present in nearly traceamounts up to about 50% by weight of the hydroxyl-bearing compound, asdesired.

Preferably, though not necessarily, the foams of the present inventionmay be prepared by reacting the polyol blend with an organicpolyisocyanate in the presence of at least one amine catalyst, asurfactant and a blowing agent.

The foams of this invention are self-extinguishing polyurethane foamsmeeting the fire-retardant requirement of ASTM-D-1692-59T and afire-retardant test similar to the Bureau of Mines' Flame PenetrationTest which is conducted as follows:

A foam specimen measuring 6 inches × 6 inches × 1 inch is mountedhorizontally on a transite heat shield with a single sheet of Number 5filter paper sandwiched between the foam and the heat shield. The heatshield measures 7 inches × 7 inches × 1/4 inch and contains a 11/2 inchdiameter hole in its center. A National Welding Equipment Company, Type3A Blowpipe Torch, equipped with a size Number 2 nozzle (for gas andair) is lit and adjusted to a flame temperature of 1177° ± 15°C., asmeasured with a chromel alumel thermocouple attached to a WestonElectric Ammeter Model 301. The flame calibration is performed in a heatshield to avoid air draft flame temperature variations. After thetemperature is adjusted the readings on two differential manometers isnoted and maintained at these levels to assure flame temperaturestability. A second heat shield, without any holes, is placed over thefoam sample and the adjusted pencil tip flame placed over the sample andaligned with the hole in the lower heat shield. The upper heat shield isremoved and simultaneously a stopwatch is started. The time required toburn through the foam as indicated by the igniting or charring of thefilter paper when viewed through the hole in the bottom heat shield, isthe burn through time.

To better enable those skilled in the art to practice the inventioncontained herein the following is a representative example of preparinga flame-retardant polyurethane foam within this invention:

EXAMPLE 8

     Formulation                                                                  ______________________________________                                        Component A             Percent by Weight                                     ______________________________________                                        polyphenyl polymethylene                                                      polyisocyanate          44.5                                                  (MONDUR MR)                                                                   Component B                                                                   Polyol Blend of Example 1                                                                             38.8                                                  Component B             Percent by Weight                                     Organo-silicon surfactant                                                     (Union Carbide L-5310)  0.6                                                   Dimethyl ethanol amine  1.0                                                   Dibutyl tin dilaurate   0.1                                                   Freon R-11-B            15.0                                                  ______________________________________                                         (FREON is the trademark of E. I. duPont de Nemours and Company).         

Component B of the above formulation is prepared by mixing together eachof the ingredients sequentially in the amount specified and thenblending the resulting mixture in a suitable container with a high speedmixer. To Component B is then added the specified amount of Component A,which is the isocyanate. The mixture of the components is stirred for 10seconds at high speed on a high speed mixer, after which it is pouredinto a one-gallon paper cup. Within 21 seconds the foaming action hasvisibly commenced (cream time), within 72 seconds the top portion of thefoam is set and non-sticky (tack time), and within 110 seconds thefoaming is complete as noted by the foam no longer rising (foam time).The flame-retardancy of the foam is then tested by ASTM method 1692-59Twith the following results:

    Seconds          Inches                                                       ______________________________________                                        25               3/4                                                          28               3/4                                                          29               7/8                                                          ______________________________________                                    

The foregoing results indicate that the foam formulation is aflame-retardant foam.

The following are representative examples of foam formulations which maybe produced by following th general procedure of Example 8. The order ofmixing the ingredients of Component B is immaterial.

EXAMPLE 9

    Component A          Percent by Weight                                        ______________________________________                                        polyphenyl polymethylene                                                                           45.5                                                     polyisocyanate                                                                (MONDUR MR)                                                                   Component B                                                                   Polyol Blend of Example 2                                                                          37.8                                                     L-5310 Surfactant    0.6                                                      Dimethyl Ethanol Amine                                                                             1.0                                                      Dibutyl Tin Dilaurate                                                                              0.1                                                      Freon R-11-B         15.0                                                     Self-Extinguishing Properties-                                                ASTM D-1692 Test:                                                             Time (seconds)                                                                            23         22         23                                          Inches      0.75       0.75       0.875                                       ______________________________________                                    

EXAMPLE 10

    Component A         Percent by Weight                                         ______________________________________                                        PAPI                38.0                                                      Component B                                                                   Polyol blend of Example 3                                                                         45.1                                                      Organo-Silicon Surfactant                                                     (Dow-Corning DC-195)                                                                              0.6                                                       Dimethyl Ethanol Amine                                                                            1.0                                                       Triethylene Diamine (33% in                                                   dipropylene glycol) 0.3                                                       Freon R-11-B        15.0                                                      ______________________________________                                    

EXAMPLE 11

    Component A         Percent by Weight                                         ______________________________________                                        MONDUR MR           45.3                                                      Component B                                                                   Polyol Blend of Example 4                                                                         38.8                                                      Organo-Silicon Surfactant                                                     (Dow Corning DC-195)                                                                              0.6                                                       Triethylene Diamine (33% in                                                   dipropylene glycol  0.3                                                       Freon R-11-B        15.0                                                      ______________________________________                                    

EXAMPLE 12

    Component A         Percent by Weight                                         ______________________________________                                        PAPI                42.8                                                      Component B                                                                   Polyol Blend of Example 6                                                                         40.3                                                      Dow-Corning-195 Surfactant                                                                        0.6                                                       Dimethylethanol Amine                                                                             1.0                                                       Triethylene Diamine (33% in                                                   dipropylene glycol) 0.3                                                       Freon R-11-B        15.0                                                      ______________________________________                                    

EXAMPLE 13

    Component A         Percent by Weight                                         ______________________________________                                        MONDUR MR Isocyanate                                                                              54.0                                                      Component B                                                                   Polyol Blend of Example 7                                                                         29.0                                                      L-5310 Surfactant   1.0                                                       Triethylene Diamine (33% in                                                   dipropylene glycol) 1.0                                                       Freon R-11-B        15.0                                                      ______________________________________                                    

EXAMPLE 14

    Component A         Percent by Weight                                         ______________________________________                                        PAPI Isocyanate     46.5                                                      Component B                                                                   Polyol Blend of Example 8                                                                         36.5                                                      L-5310 Surfactant   1.0                                                       Triethylene Diamine (33% in                                                   dipropylene glycol) 1.0                                                       Freon R-11-B        1.0                                                       ______________________________________                                    

It will be evident that although this invention has been described withreference to specific polyol blends, the substitution within this blendof any second polyhydroxy material which together with a DBBD in theproportions indicated supra achieves a hydroxyl number within thespecified range is a polyhydroxy blend within the scope of theinvention, and such interchange and modification to form a large varietyof equivalent polyol blends useful in the preparation of fire-retardantpolyurethane foams is contemplated.

Having thus described our invention, we claim:
 1. A flame-retardantpolyurethane foam composition formed in the presence of a blowing agentwhich comprises the product formed by the reaction of an organic di- orpoly-isocyanate and a polyol blend comprised of a mixture of2,3-dihalo-2-butene-1,4-diol and a polyhydroxy compound selected fromthe group consisting of sorbitol, mono- and di-saccharides andalkoxylated sorbitol and mono- and di-saccharides containing up to 10oxyalkylene or oxyarylene groups per hydroxyl groups of the sorbitol andmono- and di-saccharide wherein the hydroxyl number of said blend isfrom about 300 to about 600 and said 2,3-dihalo-2-butene-1,4-diol ispresent at a concentration of from about 15% to 40% based on the weightof the polyol blend.
 2. A flame-retardant polyurethane foam compositionof claim 1 wherein the polyol blend is comprised of2,3-dibromo-2-butene-1,4-diol and polyoxypropylene sorbitol wherein theoxypropylene content is from 2 to 20 mols per mol of sorbitol.
 3. Aflame-retardant polyurethane foam composition of claim 1 wherein the2,3-dihalo-2-butene-1,4-diol is 2,3-dibromo-2-butene-1,4-diol.
 4. Aflame-retardant polyurethane foam composition of claim 1 wherein the2,3-dihalo-2-butene-1,4-diol is 2,3-dibromo-2-butene-1,4-diol which ispresent at a concentration of about 25 percent based on the weight ofthe polyol blend.
 5. A flame-retardant polyurethane foam composition ofclaim 1 wherein the polyol blend is comprised of dibromobutenediol andpolyoxypropylene sucrose wherein the oxypropylene content is from 2 to20 mols per mol of sucrose.